Adverse effects of chronic marijuana use on learning, memory, and cognition are of serious concern in view of the widespread use of this drug, especially as use has extended into progressively younger age groups in recent years. Surprisingly, although acute impairment of memory from single marijuana doses has been frequently demonstrated, experimental studies of chronic marijuana effects on learning, memory, and cognition have been few in number, methodologically weak, and ambiguous in outcome. The first portion of the proposed study will examine effects of chronic illicit marijuana use by comparing "heavy" chronic users, "light" chronic users, and non-users matched on intellectual functioning before the onset of marijuana use. Under non-drug conditions, laboratory tests that emphasize learning and remembering new information, concept formation, associative processes, semantic memory retrieval, and psychomotor performance will be administered together will standardized tests that emphasize utilization of previously (pre-experimentally) acquired information. The study will determine whether chronic marijuana users are impaired relative to matched non-users on these tests and, if so, whether the impairments depend on the frequency of chronic marijuana use. In a subsequent portion of the study, the same tests will be administered following smoking of active or placebo marijuana to determine whether the sensitivity of individual test to chronic marijuana use corresponds with their sensitivity to acute effects of marijuana.